Out of battery discharge. I also had one with my gun. It blew out the side/base of the cartridge case, and the gas pressure destroyed the magazine, but otherwise did no damage to the gun. It sent the projectile halfway down the barrel, where it stopped. Somehow, in the excitement, I managed to pull the trigger AGAIN and now had not one, but TWO bullets stuck in the barrel.
I managed to keep all my fingers and parts of my face, and had to get creative to get those jammed projectiles out of there. I think I drilled out the one closest to the muzzle, then ramrodded the other one.
These guns are by now pretty famous for doing OOBD (also known as a "Kaboom"). Something to do with the firing pin or part of the bolt being able to hit the primer before the round gets completely chambered.
KUSA is (I think) somehow addressing the issue with some sort of a fix, but I dont know the details. This condition has been widely reported & talked about on other gun forums.
Despite this, I absolutely LOVE my KR-9. It's one of the guns I would never get rid of, even with this problem.